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Here we investigated the asymmetric transmission and the anomalous refraction introduced by a metasurface of bent gold nanowires. The refraction follows the generalized Snell’s law that takes into account the resonant behavior of metallic nanostructures located at the interface between two dielectrics. Measurements performed in the linear optical regime reveal a large sensitivity to the subwavelength features of the gold nanostructures.

We experimentally investigated the infrared properties of a set of randomly oriented silver nanowires films deposited onto glass substrate. Infrared emission of the obtained films was characterized in the long infrared range, i.e., 8-12 μm, by observing their temperature evolution under heating regime with a focal plane array infrared camera as well as a thermocouple. The obtained experimental results showed that the infrared emission from a mesh composed of silver nanowires might be tailored by opportunely assessing preparation condition, such as the metal filling factor. From the theoretical point of view, the real and imaginary part of the electrical permittivity components were retrieved from the calculations of effective permittivities of in-plane randomly oriented metallic wires, thus giving the refractive index and extinction coefficients for the four different silver nanowires meshes. Due to the correspondence between emissivity and absorbance, the experimental results are interpreted with the reconstructed corresponding absorbance spectra, thus suggesting that these coatings are suitable for infrared signature reduction applications. [ABSTRACT FROM AUTHOR]

We investigated optical limiting effects in films of tetra(tert-butyl) zinc phtalocyanine dissolved into poly(methyl methacrylate) within a 0.15% maximum weight ratio. Spectroscopic analysis was performed on the realized films in order to retrieve the corresponding linear optical absorption coefficient, whose interpolation according to a model including a set of three Lorentzian oscillators enabled the evaluation of the degrees of monomer and dimer concentrations. Thereafter, nonlinear transmission measurements were carried out at two different wavelengths, 532 and 634 nm. All the investigated films showed a nonlinear optical response, which was found to increase as the resonant condition was approached. [ABSTRACT FROM AUTHOR]

We present the results of an experimental study of an electromechanical, one-dimensional, metallodielectric photonic band gap structure. We monitor the reflectance as a function of the thickness of an air gap located inside the stack. The results suggest that, by changing the thickness of the air gap by approximately 20 nm, a large contrast can be achieved between high- and low-reflectance states, as reflectance switches from a fraction of a percent to about 70%. Using this approach, we are able to selectively remove a relatively narrow but tunable wavelength range from the reflected spectrum, thus making possible applications to accelerometers, linear optical switches, modulators, and other specialized filters in the visible range. The characteristic absorption spectrum can be controlled by varying air gap and dielectric layer thicknesses. [ABSTRACT FROM AUTHOR]

Phthalocyanines are promising molecules which present very attractive properties for nonlinear optical applications of both second and third orders. Different solutions of zinc phthalocyanine in toluene at increasing concentration were investigated by spectroscopic analysis in order to get the best conditions for the realization of films in poly(methyl methacrylate). Some films of zinc phthalocyanine in polymeric matrix were realized and their nonlinear optical behavior was characterized. The ratio by weight of the compound to the host polymer was varied between 0.004% and 0.15%. In this range all samples show a low degree of aggregation. The nonlinear optical behavior of the films was evaluated by measuring nonlinear transmission. The measurements of nonlinear transmission were carried out at 532 nm using a nanosecond Nd:YAG laser followed by an optical parametric oscillator, and a nonlinear response was observed for the two highest concentrations (0.083% and 0.15%). Finally, the nonlinear absorption coefficient β was retrieved by fitting experimental data and found to be 29 and 43 cm/GW, respectively. [ABSTRACT FROM AUTHOR]

We studied the second order optical nonlinearity of aluminum nitride films grown by sputtering onto silicon substrates. The crystalline properties of the films were investigated by x-ray diffraction measurements. Preliminary linear optical characterization of the films was carried out by spectrophotometric optical reflectance measurements at different incidence angles; thus the dispersion laws for both ordinary and extraordinary refractive indices were retrieved. Finally, second harmonic generation measurements in reflection mode were performed at a fixed angle from a fundamental beam provided by a picosecond Ti:sapphire laser system at λ=800 nm. In the experiments a high blue light conversion efficiency was found for samples 1.5 and 2 μm thick, and the second order nonlinear coefficient d33=11±1 pm/V was found. [ABSTRACT FROM AUTHOR]

In this work we investigated the second-order nonlinear optical properties of a group of Disperse-Red-1-based electro-optic fluorinated copolymers, synthesized with two fluorinated monomers: the hexafluoroisopropyl alphafluoroacrylate monomer and the alphafluoroacrylate monomer bearing the Disperse Red 1 chromophore. Copolymers, with a concentration of chromophore substituted groups ranging from 46% to 75% molar, were synthesized and deposited as single films on glass substrates, for second-harmonic generation, and as sandwich structures with electrodes, for nonlinear ellipsometry measurements. For large concentrations of substituted groups, cracking of the films was observed and did not allow for any electro-optic characterization by nonlinear ellipsometry. The second-harmonic generation tensor main component d33 was retrieved by means of second-harmonic generation at λ=1064 nm for each concentration. The results obtained for the copolymer with the lowest concentration are compared with those obtained by nonlinear ellipsometry at λ=1550 nm and used as a calibration to infer information on the electro-optic properties at large concentrations. Stability of the poling-induced second-order nonlinear properties was checked by means of nonlinear ellipsometry. By using the temperature scanning technique, a depolarization temperature Tdep=124 °C was measured, while the use of the isothermal relaxation technique allowed to observe two different decay regimes, a Vogel-Fulcher-Taman-Hesse one for temperatures above 110 °C and an Arrhenius one for temperatures below 110 °C, with activation energy EA≈58 kcal/mol. [ABSTRACT FROM AUTHOR]

We investigated second order optical nonlinearity of zinc oxide thin films, grown on glass substrates by the dual ion beam sputtering technique under different deposition conditions. Linear optical characterization of the films was carried out by spectrophotometric optical transmittance and reflectance measurements, giving the complex refractive index dispersion. Resistivity of the films was determined using the four-point probe sheet resistance method. Second harmonic generation measurements were performed by means of the Maker fringes technique where the fundamental beam was originated by nanosecond laser at λ=1064 nm. We found a relatively high nonlinear optical response, and evidence of a dependence of the nonlinear coefficient on the deposition parameters for each sample. Moreover, the crystalline properties of the films were investigated by x-ray diffraction measurements and correlation with second order nonlinearity were analyzed. Finally, we investigated the influence of the oxygen flow rate during the deposition process on both the second order nonlinearity and the structural properties of the samples. [ABSTRACT FROM AUTHOR]

The optical limiting properties of a one-dimensional, transparent metallodielectric photonic band gap structure are studied. Due to light confinement in the structure that enhances the nonlinear response of the layers, a nonlinear transmission dependence on the incident light intensity is found. Experimental results are reported for a four period sample where the single period consists of ZnO and Ag layers 109 and 17 nm thick, respectively. The structure was designed to exhibit a transmission resonance at 532 nm. Under the action of a Q-switched frequency doubled Nd:yttrium–aluminum–garnet laser, a decrease in transmission of approximately 50% is obtained for a maximum incident light intensity of 2 GW/cm[sup 2]. These results are explained in terms of a dynamic change of the absorption coefficient due to the enhancement of the two-photon absorption process. These results suggest that the structure is suitable for optical limiting applications in the visible range. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We investigate thermally induced transmission variations in a 3.6-µm-thick ZnSe/MgF[sub 2] photonic band gap structure by means of a pump-probe setup, in the 600-700 nm range, under cw pump conditions. An induced temperature increase is responsible for the thermal expansion of the layers, as well as changes in the index of refraction. As a result, the band gap structure is redshifted by several nanometers. The initial transmission of the probe beam was restored following the removal of the pump laser, thus indicating the reversible nature of the process. [ABSTRACT FROM AUTHOR]

The problem of the photothermal modulation of optical beams passing through multilayer films is an extremely complex one owing to the inhomogeneously modulated refractive index combined with multiple optical reflections inside the sample. This problem has so far not been given an exact analytical treatment in the field of photothermal probing. We consider here such a treatment for normal-incidence optical probing in reflectance of photothermally modulated single-layer thin-film samples with arbitrary optical constants. The validity of the method is demonstrated by application to a thin transparent film of silica on a silicon substrate. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In the present study, we focus into the infrared radiation (IR) absorption coefficients of several polymer types to propose a novel approach through the development of radiative-cooling polymeric films and composites. We will firstly describe the process of samples preparation. Afterwards, we will introduce the experimental setup employed for linear optical characterization in the infrared range along with the obtained experimental results. Finally, we will give details about the numerical model that we developed in order to evaluate the radiative cooling effectiveness of the different films.

We present narrow-band polarization-sensitive reflectance of GaN/AlGaN heterostructures in the mid-infrared range. Experimental measurements performed at 15° angle of incidence show the excitation of a Berreman mode at the interface between GaN and sapphire substrate. A transfer matrix method for anisotropic layers has been used to analyze the obtained results. The contribution of the two-dimensional electron gas at the interfaces of the heterostructures has been included by proper modelization of an effective thin layer.

We performed infrared optical characterization of polycrystalline β-Ga2O3 films, in the 10-18 μm range, deposited by metal organic chemical vapor deposition on sapphire substrates. Our results show that it is possible to obtain a dominant β-phase film, with a well-defined, normal to surface z-axis orientation. These results are confirmed by reflection spectra performed at 45° incidence angle which reveals a z-phonon Reststrahlen band as a function of the incident field linear polarization.

We investigated infrared reflectivity of undoped and Tungsten (W) doped Vanadium dioxide (VO2) films at varying temperatures. Undoped VO2 exhibited a clear phase transition at 100°C, achieving near 0% reflectivity, or perfect light absorption. As W doping concentration increased, the phase-transition temperature decreased, maintaining the zero-reflectivity condition. Only a 0.75% W doping enabled room temperature perfect absorption without heating the film.